Most electronic devices such as, for example, computers, televisions, radio receivers and amplifiers include electronic circuits formed by printed circuit boards. The art of printed circuit board fabrication which arose from the efforts of Strong et al, as disclosed in British Patent No. 690,691, has had a profound impact upon modern society. Ever since the initial efforts by Strong et al it has been an ongoing goal of the printed circuit board industry to increase the number of electrical circuit components which can be provided on a given circuit board surface area.
The prior art provides a method of producing a printed circuit board wherein a piece of conductive metallic foil (e.g., copper (Cu) foil) is laminated to a dielectric substrate or insulative support to produce what is known in the prior art as a printed circuit laminate blank. Generally, the dielectric substrate or insulative support is produced by impregnating woven glass reinforcement materials with partially cured resins, such as epoxy resins. The metallic foil is laminated to the insulative support by placing the metallic foil and the support into a laminating press which subjects the foil and support to elevated temperatures and pressures to fully cure, thereby bonding the foil to the support to produce a laminate comprising a resin layer having bonded thereto the metallic foil.
Generally, the metallic foil is produced by electrochemical processes in a cell containing an anode, a cathode, an electrolyte solution containing metal ions, and a source of current. Through the application of voltage between the anode and the cathode the deposition of the metal in solution is effected upon the cathode surface. The surface of the metallic foil which forms opposite to the surface which is in contact with the cathode surface includes a matte finish. During the process of laminating the metallic foil to the support this matte surface is placed in contact with the support.
Preferably, prior to the laminating process the matte surface of the foil is subjected to a process which involves the deposition of a bonding or anchoring treatment on the matte surface. This treatment increases the surface area of the foil and enhances the bondability (i.e., mechanical and chemical bonding) of the foil in order to promote adhesion with the support. The matte surface may also be subjected to additional treatments including plating or depositing thereon a thermal barrier layer to help prevent degradation of the bond between the metal foil and the insulative support during subsequent processing of the printed circuit board. In cases where copper foil is used to produce the printed circuit laminate blank, such additional treatments may include the deposition or plating of a binary alloy consisting of zinc (Zn) and tin (Sn) which serves as a barrier layer against copper migration into the resin support of the laminate as disclosed in Morisaki U.S. Pat. No. 4,049,481.
Once the laminate blank has been formed, it is then coated with a photosensitive polymer film or photoresist as disclosed in U.S. Pat. No. 3,469,982. Then, either a photographic negative or positive (hereinafter referred to as a "photographic tool") is placed on top of the laminate and in intimate contact with the photoresist. Light of the proper frequency is then caused to strike the areas of the photoresist which are not shielded by the photographic tool. Irradiation with the light causes a reaction in the photoresist which causes the exposed areas to become insoluble in a special liquid chemical or developer. The irradiated board is then developed by soaking the board in the developer which causes the unexposed or non-irradiated portions of the photoresist to be washed away, leaving a bare metallic foil surface in certain areas, the dimensions of which are controlled or dictated by the pattern of the photographic tool.
The exposed metallic foil is then removed by a suitable chemical etching solution. After stripping away the remaining photoresist, a semi-finished circuit board results having conductive and insulative areas in certain desired positions.
The above described prior art method of producing a printed circuit board presents at least one drawback. More particularly, the conductive lines or paths formed on the insulative support usually do not possess rectangular dimensions. Specifically, from the cross-sectional perspective, the conductive lines are curved inwardly due to the undercut of the chemical etching solution utilized to remove the areas of the metallic foil that are not protected by the photoresist. The degree to which this undercutting occurs influences the minimum size possible for manufacturing conductive lines on the layer of resin.
One method by which the phenomena of undercutting the photoresist with the etching solution may be minimized is by the use of a thinner layer of conductive foil in the make-up of the laminate blank. The thinner the conductive foil, the greater will be the resolution or number of conductive paths per given area. However, a point is reached when the conductive foil is made too thin to carry sufficient electrical current. In such cases, it becomes necessary to use the technique of electroplating the conductive areas with the desired metal after the conductive and insulative areas of the printed circuit board have been defined, until an acceptable thickness is reached.
Another prior art method of producing circuit boards is discussed in "Background Of The Invention" section of Barhle et al U.S. Pat. No. 4,705,592. This method includes laminating layers of copper (totalling about 75 microns) to both sides of an insulative support or substrate. Approximately 70 microns of the copper are then removed by etching. The circuit board is then cleaned and a photoresist applied and developed. Copper is then electrolessly deposited to form the circuit lines on the circuit board and tin is then deposited on the circuit lines. The remaining photoresist is removed and the copper which has not been plated with tin is then etched away. The protective tin coating is then removed thereby rendering a finished circuit board. One disadvantage presented by this prior art process is that the electroless deposition of the copper to form the circuit lines is very slow and expensive. Also, if not applied properly, the electrolessly deposited copper can be quite brittle. Additionally, precisely removing the 70 microns of copper so as to leave 5 microns of copper on the surface of the substrate can be difficult.
Barhle et al also discloses another prior art method for producing circuit boards which employs differential etching. More particularly, such prior art method includes blanket sputtering copper onto an insulative substrate. Photoresist is then applied and developed. Copper is then plated upon the exposed sputtered copper. The remaining photoresist is then stripped and the exposed sputtered copper is removed by differential etching. This prior art method presents some disadvantages in that the acid utilized to remove the sputtered copper also tends to attack the plated copper. Also, sputtering of copper is generally more expensive than plating or laminating, and sputtered copper tends to display poor metallurgical properties (e.g., it can be brittle or poorly bonded to the substrate).